Thermal oxidation of conjugated diene-based elastomers occurs during high temperature high shear mixing of the elastomers to result in a formation of oxidative components on the elastomer which are randomly distributed along the polymer chain. Representative of such resulting oxidative components are, for example, various aldehydes, ketones and epoxides.
It is a purpose of this invention to utilize such thermally oxidation formed components to form functionalization of such elastomers with functional groups pendent from and distributed along the elastomer chain.
To achieve such functionalization it is proposed to react a compound with such oxidative components on the elastomer chain having a general structure, or formula: (I):X—R—Y   (I)where X represents a group reactive with at least one of said oxidative components on said elastomer chain, Y represents a group reactive with hydroxyl groups (e.g. silanol groups) on precipitated silica (a synthetic amorphous silica) and R is a group connecting X and Y.
Representative examples of X are, for example, primary amines, secondary amines, alkyl hydrazines, aryl hydrazines, hydroxyl amines, carboxylic acids, aryl alcohols and aryl thiols. Representative of Y is, for example, an alkoxysilane group. In general Y can be defined as —SiZ3 where Z is comprised of OR, Cl, NMe2, SR, OC(═O)R or mixture thereof wherein R represents an alkyl group.
Representative of R is, for example, alkanediyl, benzenediyl, and cycloalkanediyl groups.
A representative example of such compound for such elastomer functionalization is, for example, an organoaminoalkoxysilane.
For such compound, its amine component (X component of formula I) can react with the aforesaid oxidation formed components of the elastomer and its alkoxysilane component (Y component of formula I) can react with hydroxyl groups (e.g. silanol groups) of the precipitated silica to promote rubber-silica interaction with its organo portion (the R component of formula I) being, for example, propanediyl. In this manner, then a coupling of the precipitated silica to the elastomer can be promoted to create a reinforcement effect of the precipitated silica for the elastomer.
It is considered that such created elastomer-precipitated silica interaction can be used in addition to or as an alternative to use of a sulfur-containing organoalkoxysilane compound (e.g. bis(3-triethoxysilylpropyl)) polysulfide having an average in a range of from 2 to 4 connecting sulfur atoms in its polysulfidic bridge)) which have heretofore been used as reactive coupling agents between diene-based elastomers and precipitated silica for promoting rubber-silica filler interaction and thereby various improved physical properties of rubber compositions.
Historically, use of aminosilane compounds has been previously proposed in combination with sulfur-containing organosilicon compounds for silica filled conjugated diene-based rubber compositions. For example, see U.S. Pat. No. 5,698,619.
However, for this invention, it has been discovered that a specialized procedure, or method, of functionalizing the conjugated diene-based elastomer(s) in situ within the rubber composition with an organoaminosiloxane can enable a more effective functionalization of the elastomer.
It is to be appreciated that in the manufacture of various rubber articles, rubber compositions (e.g. rubber compositions containing conjugated diene based elastomers) typically contain at least one of rubber reinforcing carbon black and precipitated silica filler reinforcement to attain desired physical and chemical characteristics. The interaction between rubber and the reinforcing filler (rubber-filler interaction) in such rubber compositions has a profound effect on the physical properties of vulcanizates (the vulcanized rubber composition). The interaction between the rubber and the filler regulates the degree of dispersion of the filler, the formation of elastomer-filler interface, and the filler-filler network within the rubber composition. All of these interactions have a significant effect on the physical properties of the cured rubber composition, such as, for example, stress-strain properties, energy loss under dynamic cyclic load, abrasion resistance, and tear propagation resistance. Increased rubber-filler interaction promotes dispersion of the filler within the rubber composition to a greater degree to thereby promote a higher level of rubber reinforcement. It can also promote an incorporation of higher amounts of the reinforcing filler within the rubber composition of which disperse into conventional rubbers with difficulty.
The importance of attaining better rubber/filler interaction has been appreciated for many years and has been the subject of numerous research projects throughout the rubber industry and within academic settings. Attaining improved rubber/filler interaction is of particular interest to manufacturers of rubber products such as tires, hoses, power transmission belts, conveyor belts, windshield wiper blades, and a multitude of other industrial rubber products and consumer goods.
One recognized approach for attaining better compatibility between rubbery polymers and fillers is to initiate or terminate polymerization using an initiator or a terminating agent which contains a filler reactive group. This approach is largely limited to anionic living polymerization.
Another recognized approach is to use a small amount of comonomer which carries a filler reactive group in the synthesis of the desired polymer.
Both of such approaches have several drawbacks. The functional initiator, terminating agent or monomer has to be synthesized and it has to be separated from byproducts if a high purity requirement of a polymerization process is desired. Functional group carrying initiators or monomers have to be stable at the polymerization temperature and should not cause undesired chain transfer or termination reaction. Functional monomers have to a have a suitable reactivity with the other monomers in order to incorporate them in a uniform manner into the polymer chain and it is usually desired that they not slow down the polymerization process significantly. Frequently a portion of the functional initiator, terminating agent or monomer has to be protected and after the synthesis of the polymer the protecting groups have to be removed. Functional groups should be stable during the finishing as well as the storage of the polymer. Therefore it is highly desirable to develop a method which permits the functionalization of the polymer after polymerization and preferentially in situ within the rubber composition during the mixing of the polymer with the reinforcing filler, particularly precipitated silica, and other ingredients.
Without intending to be bound by theory, as previously indicated, it is believed that the in situ elastomer functionalization method of this invention is based on the following reaction mechanism. Attachment of precipitated silica filler reactive aminosilane groups to the elastomer chain is based on two consecutive steps. In the first step oxygen containing reactive groups, such as one or more of aldehydes, ketones and epoxides, form along the chain due to the thermal oxidation of the polymer in the mixer via a free radical mechanism. Subsequently the amino group of the organoaminoalkoxysilanes chemically reacts with these oxygen containing entities resulting in elastomer chain bound alkoxysilane groups. Parallel with these reactions, condensation of the alkoxysilane groups with the hydroxyl groups on the precipitated silica surface takes place. The end result of these consecutive and parallel reactions is the formation of chemical bond between the elastomer chain and the precipitated silica surface to thereby promote the elastomer-filler interaction.
As free radicals and peroxides are considered to actively play an important role in the formation of the amine reactive oxygenated structures to promote the aforesaid elastomer-filler interaction, it is important that certain stabilizers should not be present during the aforesaid elastomer functionalization reaction. Representative examples of such unwanted stabilizers during the amine-based elastomer functionalization are stable free radicals such as, for example, 2,2-diphenyl-1-picrylhydrazyl (DPPH) or 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), nitroxy radical forming amine or hindered amine type stabilizers such as N-1,3-dimethyl butyl, N′-phenyl paraphenylene diamine (6PPD) or derivatives of 2,2,6,6-tetramethyl piperidine. Peroxide decomposing secondary stabilizers should also be avoided such as tris-nonylphenyl phosphites or thioethers. Contrarily, on the other hand, phenolic type (phenolic based) stabilizers have no significant effect on the functionalization reaction. Representative of such phenolic stabilizers is butylated hydroxytoluene (BHT). This may be due to, for example, a decomposition of alkylperoxycyclohexadienones at mixing temperatures exceeding 110° C.
In one aspect, as previously indicated, preparation and use of aminosilane functionalized elastomers are proposed in U.S. Pat. No. 5,698,619 for use with silica-filled rubber compositions.
However, it has been discovered that primary and secondary amines can disadvantageously and significantly interfere with functionalization of elastomers with organoaminoalkoxysilanes in a sense of retarding the functionalization process itself and, also, in the sense of the primary and secondary amines competing with the amino group of the aminosilane for preferentially reacting with the elastomer(s). Such primary and secondary amines particularly include amine type (amine based) stabilizers (antidegradants) used in rubber compositions such as, for example, N-1,3-dimethyl butyl, N′-phenyl paraphenylene diamine.
For this invention, functionalization of elastomers with organoaminoalkoxysilanes is to be accomplished in the absence of primary and secondary amine compounds either by pre-forming the organoaminoalkoxysilane functionalized elastomer(s) prior to addition to the elastomer or, more desirably in an alternative, by reacting the organoaminoalkoxysilane with the elastomer(s) in situ within the rubber composition, in the absence of primary and secondary amine compounds. If desired, primary and/or secondary amines may be added to the rubber composition following the reaction of the organoaminoalkoxysilane with the elastomer(s).
In the description of this invention, the term “phr” where used relates to parts by weight of an ingredient per 100 parts by weight of rubber, unless otherwise indicated.
The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” may be used interchangeably unless otherwise indicated. The terms “compound” and “rubber composition” may be used interchangeably unless indicated.